1. Field of the Invention
The present invention relates to a cutting insert removably installed in a cutting edge-replaceable cutting tool main body and a cutting tool in which the cutting insert can be removably installed.
2. Description of the Related Art
For example, Japanese Patent Laid-Open No. 2000-210809 discloses a cutting insert and a cutting tool which are conventionally known. That is, the cutting insert includes a rake face formed on an upper surface thereof, flank faces formed on side surfaces thereof, and a cutting edge formed at a crossing edge line between the rake face and the flank face. The cutting insert is externally shaped substantially like a sectorial plate and includes one cutting edge portion shaped substantially like a quarter circular arc. Furthermore, one rake face is formed only on the upper surface, and a seating surface is formed on the lower surface. The cutting insert also includes one mounting hole formed therein. In this form, only one cutting edge portion shaped substantially like a quarter circular arc is formed in the cutting insert. That is, once the cutting edge portion becomes unusable, the cutting insert is unusable.
Only one side of such a cutting insert is used, and the shape of the seating surface, formed on the lower surface, is not particularly limited. Thus, the seating surface can be formed adjacent to a ridge portion of the lower surface which lies opposite the cutting edge portion shaped substantially like a quarter circular arc. That is, between the upper and lower surfaces of the plate, the cutting edge portion shaped substantially like a quarter circular arc and the crossing edge line between the seating surface and the flank face formed on the side surface of the plate are positioned opposite each other. Thus, the seating surface is estimated to be formed as a plane spanning the entire lower surface. Furthermore, Japanese Patent Laid-Open No. 2000-210809 discloses a cutting edge-replaceable ball endmill as a cutting tool in which the cutting insert is removably installed.
For example, Japanese Patent Laid-Open No. H10-71521 (1998) discloses other examples of a conventional cutting insert and a conventional cutting tool. The cutting insert is shaped like a plate and includes a rake face formed on an upper surface thereof, a flank face formed on a side surface thereof, and a cutting edge formed at a crossing edge line between the rake face and the flank face. The cutting insert includes two cutting edge portions each shaped substantially like a circular arc. The rake face is formed only on the upper surface, and one mounting hole is formed in the cutting insert. The two cutting edge portions each shaped substantially like a circular arc are formed to be different from each other in length. Also in Japanese Patent Laid-Open No. H10-71521 (1998), the cutting edge portion is formed only on one side of the cutting insert. Thus, only this side of the cutting insert is used. Hence, a planar seating surface is formed all over the lower surface with no cutting edge portion formed thereon. That is, between the upper and lower surfaces of the plate, each of the two cutting edge portions each shaped substantially like a circular arc and a corresponding one of the two crossing edge lines each between the seating surface and the flank face formed on the side surface of the plate are positioned opposite each other. Furthermore, the profile of the cutting insert in an embodiment can have two acute vertices when at least one of the cutting edge portions each shaped substantially like a circular arc is set smaller than the quarter circular arc in length. Additionally, Japanese Patent Laid-Open No. H10-71521 (1998) discloses a cutting edge-replaceable ball endmill as a cutting tool in which the cutting insert is removably installed.
According to the cutting insert in Japanese Patent Laid-Open No. 2000-210809, only one cutting edge is formed in one cutting insert. That is, each cutting insert can be used only once. This disadvantageously increases manufacturing costs per cutting edge.
On the other hand, according to the cutting insert in Japanese Patent Laid-Open No. H10-71521 (1998), two cutting edges are formed in one cutting insert. Thus, one cutting insert can be used twice, reducing the manufacturing costs per cutting edge. However, if the cutting insert is used in a cutting edge-replaceable ball endmill, the cutting insert with the single shape may be used as two types of cutting edges; the cutting insert may use a cutting edge with a central edge passing through a rotation center and a cutting edge including no central edge and starting at a position away from the rotation center. For an insert seat of the tool main body to which a cutting insert including a cutting edge with a rotationally central edge is attached, the following operation is required to provide an insert seat wall surface with a sufficient thickness to resist a cutting force exerted during cutting: another cutting edge with no central edge is attached to the tool main body substantially away from the rotation center.
Given that in the cutting insert in Japanese Patent Laid-Open No. H10-71521 (1998), the cutting edge with no central edge is placed close to the rotation center, two cutting edge portions each shaped substantially like a quarter circular arc need to be formed on the one upper surface. In this case, the profile of the cutting insert is difficult to provide with two acute vertices. Both of the two vertices are inevitably formed to have an obtuse angle that is about 90 degrees or greater than 90 degrees (see an angle α in FIG. 15). That is, if a cutting edge shaped substantially like a quarter circular arc is placed close to the rotation center, the sum of angles of two vertices of the cutting edge is 180°. Here, if one of the two cutting edges is assumed to have an acute angle α°, the other cutting edge has an obtuse angle (180°-α°). Furthermore, if a cutting edge shaped substantially like a one-third circular arc which is longer than the cutting edge shaped substantially like a quarter circular arc is placed close to the rotation center, the sum of angles of two vertices of the cutting edge is 240°. Here, if one of the two cutting edges is assumed to have an acute angle α°, the other cutting edge has an obtuse angle (240°-α°). That is, the cutting edge portion needs to be shorter than the quarter circular arc in order to set both of the two cutting edges to have acute angles. When a cutting edge with no central edge is thus placed close to the rotation center, vertices equal to or greater than about 90 degrees lie close to each other. In this case, a part of the wall surface of an insert seat of the tool main body which part is located near the rotation center is likely to have an insufficient thickness or cannot be provided. Thus, the cutting insert cannot be fully held and is easy to move. Hence, disadvantageously, the tool offers a sharply reduced stiffness.
Moreover, the wall surface of the insert seat of the tool main body in Japanese Patent Laid-Open No. H10-71521 (1998) is mainly formed of a circular-arc wall surface portion as seen from a direction opposite to a bottom surface of the insert seat, in order to support the profile of the cutting insert. Thus, the cutting insert is easy to move in the direction of rotation, disadvantageously reducing a clamp stiffness.
Furthermore, if a cutting edge with no central edge is attached to the tool main body away from the rotation center, only the cutting insert including a cutting edge with a rotationally central edge carries out machining at the end portion of the tool during cutting. Since the cutting edge with no central edge is located away from the rotation center, a larger area needs to be machined only by the cutting edge with the rotationally central edge. When a larger area needs to be machined only by the cutting edge with the rotationally central edge, the cutting edge with the rotationally central edge needs to bear an increased load. Thus, compared to the tool life of the cutting edge with no central edge, the cutting edge with a rotationally central edge has a shortened tool life. In other cases, in order to prevent the tool life from being shortened, the level of cutting conditions needs to be decreased, disadvantageously resulting in reduced efficiency.
Thus, an object of the present invention is to provide a cutting insert and a cutting tool which require only low manufacturing costs per cutting edge and which offer a very high tool stiffness, the cutting insert and cutting tool thus enabling efficient cutting.